In hospitals containing many patients, it is common practice to preload the hypodermic syringes in the hospital pharmacy and then transfer them to the particular floors for injection into patients.
Many medicaments are supplied to the pharmacies in vials with puncturable rubber stoppers sealing an outlet of the vial. It has been common practice to attach a hypodermic needle to each syringe to be filled. Sometimes the floor nurse giving the injection required a different size needle to inject into a patient than the pharmacist used to fill the syringe, which filling needle was sent along with the syringe to the floor nurse. In these situations, two needles were needed for a single patient injection.
When a common vial was used to fill many syringes, this required multiple punctures of the rubber stopper. This could cause mechanical deterioration of the stopper material and permit tiny bits of the rubber stopper to be drawn into the syringe. Despite this disadvantage, hypodermic needles with thin tubular metal cannulae are continually being used to repeatedly puncture the vial's rubber stopper. These cannulae are used for such repeated punctures because the stopper tends to reclose the puncture passage upon withdrawal of a particular cannula, thus performing a valving function.